It is already known that nicotinic acid derivatives have herbicidal activities. For example, nicotinic acid derivatives represented by the formula ##STR2## (Published Specification WO91/10653), heterocyclic derivatives represented by the formula ##STR3## (Published Specification EP0461079) and nicotinic acid derivative represented by the formula ##STR4## (Published Specification DE4026177) are known to have herbicidal activities.
The compounds disclosed in the above-mentioned Published Specifications are not necessarily satisfactory from the viewpoint of the herbicidal effects. A number of other herbicides have been developed and contributed to labor saving in farm work and to the improvement of the productivity. However, in their practical use, such herbicides also have various problems with respect to the herbicidal effects and the safety to crop plants.
Especially, in cultivation of barley and wheat, few herbicides can control gramineous weeds congeneric to barley and wheat, such as water foxtail, black grass and annual blue grass over a broad period of time from the pre-emergence season to the growing season of the weeds. Further, few herbicides have a broad selectivity between these herbicides and barley or wheat.
The present inventors have conducted an extensive research on nicotinic acid derivatives with the object of developing compounds which are not phytotoxic to valuable crop plants and have excellent herbicidal activities. As a result, they have found that the compounds of the present invention which are pyrimidine and triazine derivatives bonded to nicotinic acid substituted with a phenyl group or with a heterocyclic ring, exhibit excellent herbicidal effects against not only annual weeds but also perennial weeds, and are highly safe to crop plants. The present invention has been accomplished on the basis of this discovery.